The process of manufacturing optical lenses, and in particular corrective ophthalmic lenses, requires a particularly high level of care and precision. It generally involves two main steps. First of all, a semi-finished lens, also referred to as a blank disk or preform, is obtained by molding the synthetic or inorganic material selected from which to make the base substrate of the lens. Next, the molded semi-finished lens is surfaced on one and/or other of its two optical faces in order to conform to the prescribed geometric model and prescribed correction.
Because of the stringent requirements regarding precision and roughness to which it is subject, this surfacing operation is broken down into several sub-steps associated with the same number of specific workstations. Thus, in general, for the surfacing of each face of the lens, a distinction is made between a machining workstation which performs both the rough-cutting and the finishing using two distinct tools, and a polishing workstation which may potentially be preceded by a smooth-grinding workstation.
One of the most specific problems encountered during the course of this lens-surfacing process lies in the mounting of the lens on each workstation with a positioning that is precise and well controlled. This recurring intermediate operation of picking up the workpiece over and over again, commonly referred to as the lens-blocking operation, is particularly tricky and expensive and often gives rise to imprecisions in positioning which are capable of significantly deteriorating the optical quality of the finished lens. In fact, this lens blocking is subject to two cumulative and opposing requirements.
First of all, the lens, made of transparent synthetic or inorganic material and not yet coated, is relatively fragile and needs to be spared any marking or cracking, especially on the one of its two faces that is finished, while its other face is in the process of being worked. The risk of marking is particularly pronounced with synthetic materials.
In addition, and above all, the lens needs to be positioned on each workstation concerned very precisely, with a spatial orientation that is known and stable in a determined frame of reference of the workstation concerned. This requirement of geometric stability of the blocking is particularly steep and difficult to meet in the case of the manufacture of lenses with complex surfaces such as varifocal or customized lenses which do not have symmetry of revolution. Indeed it will be appreciated that the surfacing of such lenses is accompanied by variations in cutting force that vary according to intense gradients thereby giving rise to deformations the result of which is relative geometric instability of the blocking of the lens.
Several ways of “blocking” a blank or semi-finished lens for mounting it and rotationally driving it on the machine tools or measurement apparatuses of the various workstations and, in particular, surfacing workstations, are known. Traditionally, use is made of a blocking support, sometimes also referred to as a holder block or chuck, which on the one hand has blocking means to accept and immobilize the lens via one of the main faces thereof and on the other hand has means for fixing this support to the tip of the various machine tools or measurement and control apparatus, so as to block the lens, if appropriate allowing it to be rotationally driven, on the machine or the apparatus.
The main difficulty lies in how to block the lens on this support, because of the requirements mentioned hereinabove.
The most widespread method in use at the present time, because of its geometric precision, is to use casting to cast a low melting point molten alloy onto one of the faces of the lens to form a metallic block that acts as a blocking support and that has means of fixing it to the tip of the machine tools of the various workstations involved.
This method is satisfactory overall in terms of its precision and stability, but has a number of disadvantages of an economic and environmental nature that make it necessary to look for alternative blocking means. The low melting point alloys used are in fact relatively high in cost and need to be considered as pollutants that are hazardous to the environment, which means that both for economical reasons and because of increasing environmental requirements, it is necessary to organize the recycling thereof very carefully. However, even with effective recycling, losses of alloy through evaporation at the time of melting cannot be avoided. There is also, for technical reasons, a minimum length of time (approximately 15 minutes) that needs to elapse before the lens associated with its holding block can be used on a machining workstation, and a maximum length of time (approximately 24 hours) beyond which the machining will no longer be able to be carried out; these times therefore impose requirements on the workflow of said lenses.
In order to avoid the use of a molten metal alloy, the idea of using, for example a wax, to bond the lens to a corresponding face of the blocking support, having approximately the same curvature, has been conceived of. However, this solution, just like the meltable metal block solution incidentally, presents practical problems regarding unblocking, namely the disassembling of the lens from the support, and the cleaning of the lens, with the ensuing environmental repercussions. Above all, the precision and stability with which the lens is fixed to the support may prove insufficient. The geometry of the layer of adhesive or wax interposed between the lens and the support in fact takes on a haphazard nature or in any case is difficult to control and may experience deformations, in compression and torsion, during the surfacing operations under the effect of the stresses generated by the surfacing tool.
Finally, lens-blocking systems using a pneumatic depression or reduced pressure have been proposed. Such systems use a holder block or pneumatic chuck which, in order to form a kind of controlled-operation vacuum cup, have a cavity bordered by an annular seal against which the preform is brought to bear in order with the cavity and the seal to delimit a chamber in which a relative vacuum is created. The vacuum may be created either in a box containing, for the blocking operation, the holder block and the lens or under the effect of a vacuum pump connected to the cavity of the block via a pneumatic valve.
This pneumatic blocking solution, also referred to as vacuum blocking, does not have the same economic and environmental disadvantages as the cast or bonded block solutions mentioned previously. Use of this solution is, in effect, particularly simple and quick, both at the blocking and at the unblocking stages, and requires no chemical consumable. However, despite these considerable advantages, this type of blocking is very little used in practice. This is because a lack of precision and stability of the fixing of the lens similar to that observed with the bonded supports is observed. The solution in particular proves difficult to implement for complex surfaces (surfaces other than spherical or toric surfaces) with respect to which the elastically compressible seal does not bear sufficiently precisely and stably. Admittedly, it is then possible to consider increasing the compression stiffness of the seal, but that is at the expense of its coefficient of friction, resulting in a lowering of the torque transmitted for the rotational drive of the lens. This is unless the pressure in the reduced-pressure chamber is reduced in order to increase the intensity of the suction-cup effect applied by the support to the lens, which would carry the risk of deforming the latter.
Document FR2863520 therefore discloses a pneumatic blocking support comprising a central cavity and, around the latter, an annular seal against which the lens is brought to bear. Three projecting pins are provided on the support, on each side of the annular seal, to form a tripod designed to offer the optical lens a rigid footing after the seal has been elastically compressed.
The firmness of the footing of the lens on the tripod thus provides the stability and precision of the geometric positioning of the lens on its support.
The major disadvantage with this solution is that it is not suited to all shapes of lens, notably to lenses the optical faces of which are not very curved and which press against the tripod before the seal is correctly compressed.
These lenses are then found to be held against the seal rather haphazardly.
It is also found that the blocking forces applied to the lens are poorly distributed. The surfacing of the parts of the lens which are situated distant from these pins therefore causes lens deformation that is detrimental to the precision of the machining.